The present invention relates to the art of casting and more particularly to the art of casting metals using a mold or core produced by the investment casting method.
Metal is cast by pouring the molten metal into a mold, possibly containing a core, made of a porous or particulate material such as sand. Other materials can be cast by similar means using molds of known porous substances such as plaster or even paper fiber. One popular method of preparing molds or cores for casting is the "investment casting" method.
In an investment casting process, a model of the item to be cast, called a pattern, is shaped from a destructible material such as polymer foam. The mold or core is shaped from sand around the pattern. The polymer pattern is then destroyed by a method which leaves the mold or core bearing its impression intact. Such processes are described in Horton, Method of Form Removal from Precision Casting Shells, U.S. Pat. No. 3,094,751 (June 25, 1963); Moxlow, Metal Casting Using Destructible Pattern, U.S. Pat. No. 3,226,785 (Jan. 4, 1966); Poe, Expandable Molding Shape for Precision Casting, U.S. Pat. No. 3,254,379 (June 7, 1966); Horton, Method of Removing Patterns from Investment Molds, U.S. Pat. No. 3,339,622 (Sept. 5, 1967); Bayer, Casting Method, U.S. Pat. No. 3,410,942 (May 24, 1968); Snelling, Mold for the Casting of Metals, U.S. Pat. No. 3,526,266 (Sept. 1, 1970); Burkett et al., Process for Making Soluble Cores, U.S. Pat. No. 3,587,435 (Dec. 31, 1974); Trumbauer, Casting Methods with Composite Molded Core Assembly, U.S. Pat. No. Re. 31,488 (Jan. 10, 1984); Trumbauer, Casting Methods with Composite Molded Core Assembly, U.S. Pat. No. 4,462,453 (July 31, 1984).
The pattern can be destroyed by known methods such as melting, decomposition or contacting with molten metla, but a preferred method is to contact the pattern with a solvent capable of dissolving it. See supra Horton U.S. Pat. No. 3,094,751, Poe U.S. Pat. No. 3,254,379, Horton U.S. Pat. No. 5,339,622, Bayer U.S. Pat. No. 3,410,942 and Trumbauer U.S. Pat. No. 4,462,453. When the mold or core is contacted with solvent, some solvent is drawn into it and becomes adsorbed upon the sand or other mold or core material. The presence of solvent in the mold or core is undesirable for several reasons. When casting metal, the heat from the molten metal causes the solvent to form a gas, which can crack the mold or core or cause bubbles in the cast item. Molten metal may react with the solvent, for instance by forming hydrochloric acid from chlorinated aliphatic solvents. The solvent may escape from the mold or core into the environment, posing a health threat to workers and the general public. Finally, the solvent may be too expensive to lose substantial amounts with each mold or core.
Solvent-treated molds and cores are currently dried either by heating or by subjecting to reduced pressures or both. Solvent thus driven off can be scrubbed from the entraining air by passing the mixture through carbon adsorption beds. That system has numerous faults. Used carbon beds can not be regenerated infinitely but must eventually be disposed of in an environmentally sound manner. Moreover, common chlorinated solvents such as 1,1,1-trichloroethane can react while in the bed to form hydrochloric acid which damages the adsorption equipment. Furthermore, since the efficiency of carbon beds is less than 100 percent, a system which sends large amounts of solvent to the carbon beds will lose more solvent into the environment than one which sends lesser amounts to the adsorption beds. All of those problems can be minimized by reducing the flow of solvent into the carbon beds.
What is needed is a process to produce the molds and cores which minimizes the amount of solvent retained in them and recovers a substantial portion of the solvent which is retained in a reusable fashion.